Nasal absorption of tyrosine-linked model compounds

The objective of this research is to explore the amino acid carrier-mediated pathway for nasal uptake of tyrosine-linked model compounds. These molecules were synthesized by ether linkage between the aromatic hydroxyl group of L-tyrosine and the terminal hydroxyl group of model compounds. An enzymatically stable linkage was utilized to link the model compounds to L-tyrosine so that the loss of the compound from perfusate signals absorption only without complication from metabolism. The rat in situ nasal perfusion technique was utilized in this investigation. The analyte concentrations remaining in the nasal perfusates were quantitated by a reversed-phase high-performance liquid chromatography. All L-tyrosine-linked model compounds were absorbed across the nasal mucosa (15-50% absorbed), and the extent of nasal absorption was concentration dependent. The apparent first-order rate constants of intact conjugate loss from the nasal perfusate were calculated. The Michaelis-Menten kinetic model was applied to fit the perfusion data. The Michaelis constant, K(m), and maximum or saturation rate of compound loss from the perfusate, V(max), were calculated from Lineweaver-Burk plots [e.g., K(m) and V(max) are 0.57 mM and 0.00515 micromol/min, respectively, for one of the model compounds, O-(4-carboxyphenylmethyl)-L-tyrosine]. The absorption characteristics of all these model compounds exhibited saturable kinetics, whereas their parent compounds, possessing different octanol-water partition coefficients, showed either no absorption or transport by passive diffusion. Nasal uptake of L-tyrosine conjugates was inhibited by L-tyrosine. The results from this study indicate that nasal absorption of L-tyrosine-linked compounds is probably through an amino acid transport system. Exploitation of the amino acid transport pathway may be utilized to improve nasal absorption of poorly permeable drugs.